1. Field of the Invention
This invention relates to a passive shut-down heat removal system for use in either a pool-type or loop-type liquid metal cooled nuclear reactor.
2. Description of the Prior Art
Systems for removing the decay heat within pool-type liquid metal reactors in the event of a shut-down condition are known in the prior art. Generally, such heat removal systems include an auxiliary heat exchanger formed from a loop of stainless steel piping that is formed into coils at either end, and includes a heat exchange medium, such as a mixture of liquid sodium and liquid potassium. The coils at one end of this loop of pipe are immersed in the hot pool of liquid sodium within the reactor vessel, while the coils at the other end are positioned outside of the reactor vessel and disposed within a flue. The entrance of the flue is covered by a set of louvers that may be selectively opened by electric motors. Such auxiliary heat exchangers form the heart of what is known as a direct reactor auxiliary cooling system (or DRACS) in the nuclear engineering art. When a shut-down condition occurs in which the normal reactor heat rejection system becomes unavailable, the louvers over the flue entrance are opened by actuating the electric motors, which in turn allows a naturally created draft of ambient air to flow through the flue and over the coils of the loop of pipe that forms the auxiliary heat exchanger. thereby cooling the hot pool of sodium. Such a shut-down condition might occur, for example, as a result of the failure of the secondary system pumps to circulate liquid sodium from the intermediate heat exchangers in the hot sodium pool to the steam generators of the plant.
While such prior art shut-down heat removal systems are capable under most circumstances of removing the decay heat from the pools of liquid sodium in the event of such a pump failure or other emergency shut-down condition, problems could arise in circumstances where the nuclear plant facility experienced a total blackout of electrical power that rendered the louver motors inoperative, or in a case where the louvers malfunctioned and became stuck in the closed position. Additionally, the structure of these auxiliary coils creates other problems in the over-all design of the reactor facility. However, before these problems may be fully appreciated, some understanding of the structure and operation of pool-type nuclear reactors is necessary.
Pool-type liquid metal reactors are generally formed from a reactor vessel formed from stainless steel that is in turn circumscribed by a guard vessel. The bottom of the reactor vessel is filled with a pool of liquid sodium that has a temperature of approximately 670.degree. F., while the middle and upper portions of this vessel are filled with a pool of liquid sodium that is heated to a temperature of approximately 950.degree. F. Hence the top and bottom pools within the reactor vessel are known as the hot and cold pools, respectively. A support plate spaced from the bottom wall of the reactor vessel defines a pressure boundary between the hot and cold pools of liquid sodium. Below the hot pool, a nuclear core is centrally supported on the upper surface of the support plate and completely immersed in liquid sodium. Also included in the hot pool are a multiplicity of primary pump standpipes as well as intermediate heat exchangers. The bottom end of each primary pump standpipe is attached to the support plate and includes both an inlet and an outlet conduit. The inlet draws sodium from the cold pool out of the cold pool plenum formed between the support plate and the bottom of the reactor vessel. The outlet conduit directs a relatively cold flow of liquid sodium into the entrance of the nuclear core in order that it may be circulated through a bank of nuclear fuel rod assemblies before being injected into the hot pool above. The intermediate heat exchangers in the hot pool circulate a flow of hot sodium to a secondary heat exchange system, which is ultimately used to generate nonradioactive steam to turn the blades of a turbine attached to an electric generator. The bottom end of the intermediate heat exchanger includes an output port that communicates with the pool of cold liquid sodium, and liquid sodium whose heat energy has been transferred into the intermediate heat exchange system flows through this outlet port to replace the liquid sodium that is constantly being sucked into the primary pump.
Under normal operating circumstances, the primary pump and the intermediate heat exchangers maintain the liquid sodium in the hot pool at a temperature of approximately 950.degree. F. At such a temperature, the free surface of the liquid sodium that forms the hot pool is maintained at a predetermined design level that is well below the upper edge of the reactor vessel. However, if the normal heat rejection system should fail, most of the decay heat generated by the nuclear core would be absorbed within the hot pool of sodium. If no shutdown heat removal system is included within the design of the pool-type reactor, the decay heat could raise the temperature of the liquid sodium in the hot pool well above its design temperature, which would cause it to thermally expand to a level that floods the bottom of the reactor vessel closure. The excessive high temperature would also damage the core fuel elements and other structures of the reactor. While the resulting sodium expansion would be contained by the reactor vessel, the resulting shut-down and repair of the reactor and clean-up of the sodium would be very expensive and hence highly undesirable.
The purpose of providing a shut-down heat removal system in a pool-type liquid metal reactor is to prevent the temperature of the liquid sodium in the hot pool from rising to a level where thermal damage is incurred by the reactor core and structural members. Unfortunately, prior art shut-down heat removal systems that are dependent upon electrically powered components, such as the previously mentioned louver motors, may fail to operate in the event of a total electrical power failure. In an attempt to minimize the risk of such failure, some of the systems which utilize movable louvers are designed so that they may be manually cranked open. However, such a shut-down heat removal system is still susceptible to failure in the event of a plant accident that simultaneously injures the plant operators while causing a power failure. Still another shortcoming of this system arises from the fact that the coils of the auxiliary heat exchanger are mounted on top of the intermediate heat exchanger that stands in the hot pool of the reactor. Such positioning of these coils necessitates a lengthening of the height of the reactor vessel of about eight feet, which in turn substantially increases the construction cost of the reactor facility. Moreover, such a design is limited to pool-type liquid reactors, since the intermediate heat exchangers of loop-type metal reactors are positioned outside of the reactor vessel. Finally, the fact that the immersed coils of the auxiliary heat exchanger are always transferring some heat from the hot pool to an area outside of the reactor vessel creates heat losses that undermine the efficiency of the reactor.
Clearly, there is a need for a shut-down heat removal system that is in no way dependent upon electric motors or human operators. Ideally, such a system should be compatible with both pool-type and loop-type liquid metal reactor designs, and easily incorporated therein without the lengthening or alteration of the reactor vessel or other major components of the plant. Finally, such a shut-down heat removal system should be fully automatic in operation, simple in construction, and high in reliability without the creation of heat losses that impair the over-all efficiency of the reactor.